Certain semiconductor devices require layered structures properly positioned on a semiconductor substrate for the intended use of the device. Examples include solid state imagers wherein the positioned layered structures are used to provide optical exposure to the device. For such imagers to be responsive to color, it is conventional to provide the imagers with color separation means. Highly preferred separation means are color filter arrays, particularly those that are integral with the underlying photosensor substrate. Particularly useful examples can be found in U.S. Pat. Nos. 4,190,446; 4,196,009; and 4,285,007, issued on Feb. 26, 1980; Apr. 1, 1980 and Aug. 18, 1981, respectively. Such integral color filter arrays are conveniently formed by coating the photosensor substrate with two or three dye-imbibable layers. The dyes can be selected to provide, e.g., blue, green and red filter elements in a predetermined pattern.
Designers of such integral color filter arrays often are faced with a dilemma. Unless the surface on which the individual filter elements are formed is planar, the arrays are subject to cracking and non-predictable performance. Cracking is unsatisfactory in a filter as it tends to allow white light through to expose the device. However, because the semiconductor circuit elements comprising the photosensor are by construction non-planar, with topography having a difference in height that is often in the range of 1.5.mu., planarity is achieved only by interposing a planarizing layer between the filter elements and the photosensors. Because most coating materials are planarizing only when coated at substantial thicknesses, i.e., 4.mu. thickness or larger, the individual filter elements are raised a substantial distance from their underlying photosensor. The greater this distance is, the more likely it is that optical cross-talk will occur, wherein light from one filter element strays laterally to a pixel element not aligned with that element. Contamination due to cross-talk is then avoided, if at all, by shielding over that area susceptible to cross-talk. However, shielding can cause a loss in photosensor speed. Furthermore, certain kinds of imagers, such as charge-coupled frame transfer devices, have the potential to use 100% of the pixel area as photosensitive area.
On the other hand, planarizing compositions such as certain particular polymers coated out of a solution, have provided insufficient planarization when coated at thicknesses designed to avoid the optical cross-talk. This is believed to be due, in part, to hardening of the layer by evaporating the solvent for the particular polymer. As the solvent evaporates, some shrinkage in the polymer apparently occurs and the coating loses some of its planarization.
Thus, it has been very difficult, prior to this invention, to find a planarizing layer that is effective in providing a planar surface for subsequent layers without spacing those layers too far away, e.g., a distance exceeding 3.mu..
Just as conventional intervening layers have been largely unable to effectively reduce their thickness, conventional methods for measuring the degree of planarization (D) have also become less adequate. Conventionally, D=(1-t.sub.s /t) for t=the maximum height of the topography of the underlying substrate and t.sub.s =the remaining topographical height in the planarizing layer arising from the underlying topography, that is, the extent to which due to the underlying topography, the planarizing layer protrudes above the plane which its exterior surface otherwise occupies.